1. Field of the Invention
The present invention relates to a diesel engine system, and more particularly relates to reducing pollutant such as NOx (nitrogen oxide) and PM (Particulate Matter) in exhaust gas from a diesel engine. This Patent application is based on Japanese Patent application No. 2007-050719. The disclosure thereof is incorporated herein by reference.
2. Description of the Related Art
Exhaust gas recirculation (EGR) in which exhaust gas from a diesel engine is partially recirculated to an intake pipe is known as one of methods used to reduce nitrogen oxide in the exhaust gas. The exhaust gas includes no oxygen or oxygen of low concentration. Thus, when the exhaust gas is recirculated to the intake pipe, combustion is carried out in a state of low oxygen concentration. For this reason, a combustion temperature is decreased, thereby suppressing the production of NOx. The exhaust gas recirculation is disclosed in, for example, Japanese Patent No. 3448862, Japanese Patent No. 3092547, Japanese Laid Open Patent Application (JP-A-Heisei, 11-50917) and Japanese Laid Open Patent Application (JP-A-Heisei, 11-280525).
A structure of a diesel engine that employs the exhaust gas recirculation is described schematically as follows. An EGR path (exhaust gas recirculation channel) is provided between an exhaust port and an intake port of the diesel engine. Along the EGR path, exhaust gas is recirculated to an intake path. An EGR valve (exhaust gas recirculation controlling valve) for controlling the flow rate of the recirculated exhaust gas is provided to the EGR path. In addition, a throttle valve (intake throttle valve) for controlling the flow rate of intake air is provided to the intake path.
The openings of the EGR valve and the throttle valve are controlled such that an excess air ratio agrees with a desired value. Specifically, when the excess air ratio is higher than the desired value, the opening of the EGR valve is made to increase, thereby making the flow rate of the recirculated exhaust gas to increase. After the EGR valve is fully opened, the opening of the throttle valve is made to decrease. In such control, the flow rate of the recirculated exhaust gas is made to increase and the flow rate of external air as the intake air is made to decrease as necessary, and thus the excess air ratio is made to decrease to the desired value. On the other hand, when the excess air ratio is lower than the desired value, the opening of the throttle valve is made to increase. When the throttle valve is fully opened and the excess air ratio is still lower than the desired value, the opening of the EGR valve is made to decrease. In such control, the flow rate of the external air is made to increase and the flow rate of the recirculated exhaust gas is made to decrease as necessary, and thus the excess air ratio is made to increase to the desired value.
It is important that the openings of the EGR valve and the throttle valve are required to be controlled based on the load of the diesel engine in order to reduce the nitrogen oxide effectively. Because the openings of the EGR valve and the throttle valve to attain the desired excess air ratio vary depending on the load of the engine. Even when the opening of the EGR valve is constant, the flow rate of the recirculated exhaust gas varies depending on the load of the diesel engine, especially, on an engine speed of the engine (number of revolutions of the engine).
Specifically, when the load of the diesel engine is small (namely, when the engine speed is low), the pressure of the intake air is low. Thus, the flow rate of the recirculated exhaust gas can be high even when the opening of the EGR valve is small. Therefore, when the load of the diesel engine is small, the opening of the EGR valve is made small, and the opening of the throttle valve is made large. On the other hand, when the load of the diesel engine is large, the opening of the EGR valve is required to be large to make the exhaust gas to be recirculated at a desired flow rate. Therefore, when the load of the diesel engine is large, the opening of the EGR valve is made large, or the opening of the throttle valve is made small. Such control can maximize the reduction of the nitrogen oxide.
Such control has one problem that much smoke is produced during the increase in the load of the diesel engine. In other words, much particulate matter is produced during the increase in the load. From the viewpoint of securing the flow rate of the recirculated exhaust gas to reduce the nitrogen oxide, it is preferable to make the opening of the EGR valve to increase as the load of the diesel engine increases. It is more preferable to make the opening of the throttle valve decrease as necessary in addition to the increase in the opening of the EGR valve. However, such control has an adverse effect that the necessary flow rate of the intake air is not secured during the increase in the load of the diesel engine. As the load of the diesel engine increases, the necessary flow rate of the intake air increases. However, the opening of the EGR valve is made to increase or the opening of the throttle valve is made to decrease in order to reduce the nitrogen oxide, the external air supplied to the diesel engine is reduced, and thus the necessary flow rate of the intake air is not secured. In such case, incomplete combustion occurs in a combustion chamber of the diesel engine, and the smoke is produced. Since the increase in the load of the diesel engine has an action to induce the increase in the flow rate of the intake air, after the end of the increase in the load of the diesel engine (namely, after the end of the increase in the engine speed), the problem is not critical that the necessary flow rate of the intake air is not secured. However, the action to induce the increase in the flow rate of the intake air is not effective during the increase in the load. Therefore, during the increase in the load, the problem is critical that when the opening of the EGR valve is made to increase or when the opening of the throttle valve is made to decrease, the necessary flow rate of the intake air is not secured and the smoke is produced.
From such background, it is required to suppress the production of the smoke during the increase in the load of the engine without inhibiting the reduction of the nitrogen oxide.
By the way, Japanese Laid Open Patent Application (JP-A-Heisei, 6-74070) discloses an air fuel ratio controller which controls air fuel ratio for an engine. A turbo charger, which is driven by exhaust gas from the engine, is connected to the engine through an intake path. The intake path is provided with a gas mixer and a throttle valve. The throttle valve is arranged between the gas mixer and the engine. The gas mixer supplies mixed gas in which fuel and intake air are mixed to the engine through the throttle valve. A portion of the intake path between the turbo charger and the gas mixer is connected through a bypass valve to another portion of the intake path between the gas mixer and the throttle valve.
The air fuel ratio controller includes a first delay circuit and a second delay circuit. The first delay circuit averages a voltage signal which indicates oxygen concentration in the exhaust gas. The second delay circuit delays a difference signal between the averaged voltage signal and a desired value signal. The desired value signal is set based on temperature of the exhaust gas. The controller controls the bypass valve based on the delayed difference signal.